Semiconductor process



SEMICONDUCTOR PROCESS Filed March 31, 1961 Core of Semiconductor Material Vupor deposition of semiconductor material 'with a predetermined concentration of impurities Doped Substrate Vapor deposition of pure semiconductor material Fl I to increase diameter and dilute impurity concentration Diluted Substrate 2one refining to produce predetermined resistivity single crystal material FIG. 2

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a? INVENTOR HEINRICH GUTSCHE BY Y J 5 M W ATTORNEY United States Patent 3,103,672 SEIVHCQNDUQTGR PRGCESS Heinrich Gutsche, Danvil e, ha, assignor to Merck 6a., Inn, Runway, Nd, a corporation of New Jersey Filed Mar. 31, 1961, fies. No. 9%,hti6 2 Claims. (Ql. 252-623) This invention relates to a method of manufacture of semiconductor materials and, more particularly, to a method of producing uniformly-doped single crystals of semiconductor materials having predetermined resistivities.

It is known in the art that the type of conductivity of semiconductor materials may be controlled by introducing certain foreign materials, generally termed impurities, into a melt of the semiconductive material. The introduction of such impurities into the semiconductor ma terials is usually referred to as doping. The type of conductivity established in the semiconductor is dependent upon the electron configuration of the atoms of the impurity material and of the host crystal. Thus, a substance whose atoms are capable of giving up electrons to the atoms of a particular substance is termed a donor impurity, and since there is a surplus of electrons available to carry a current, the semiconductor so doped is deemed to be of N-type (negative) conductivity. On the other hand, a substance whose atoms are capable of borrowing or accepting electrons is termed an acceptor impurity, and since there is a shortage of electrons in the crystal lattice available for current conduction, the semiconductor so doped is deemed to be of P-type (positive) conductivity.

The usual and most directly convenient manner hitherto achieved in the art for introducing predetermined amounts of such impurities into a semiconductor material is to add the impurity into a melt of the semiconductor material. Some semiconductor materials, however, have relatively high melting points. Silicon, for example, melts at 1420 C. Great difiiculty is therefore encountered in attempting to dope such high melting point semiconductors with lower boiling point impurities inasmuch as such impurities boil olf either prior to introduction into the semiconductor melt or before alloying with the molten semiconductor. This is especially true when the boiiing point of the impurity is greatly lower than the semiconductor melting point.

Accordingly, it is an object of the present invention to provide a method for the preparation of uniformly doped single crystals of semiconductor material having predetermined resistivities.

Still another object of the instant invention is to provide uniformly doped single crystals of semiconductor material of predetermined conductivity type and resistivity wherein the resistivity level is achieved by adjusting the concentration of impurities in a master rod in a predetermined controlled manner, then diluting the concentration by controlled deposition thereon and thereafter zone refining the master rod during a selected number of Zone passes to further reduce the impurity concentration to the desired level.

These and other objects will be made from the following more detailed description of the invention which will be made clear by the reference to the accompanying drawings.

In the drawings:

FTGURE 1 is a flow sheet of the process steps of the present invention.

FIGURE 2 is a schematic illustration of suitable apparatus for use in the invention.

In accordance with the present invention there is provided a process for the preparation of uniformly doped single crystal semiconductor material of predetermined dd'd fl 'lii Patented Get. 22, 1963 resistivity. In general, the desired resistivity level of the semiconductor material is achieved by doping a master semiconductor core of relatively small diameter to a predetermined relatively high doping level, then diluting the concentration by controlled deposition thereon of pure semiconductor material and thereafter decreasing this controlled level step-wise by vacuum zone refining, the extent of the decrease being a function of the pass speed and the number of zone passes to which the doped material is subjected.

More specially, the present process, which will be described in detail hereinafter, may be summarized by the process steps wherein there is first provided an elongated semiconductor rod in a reactor chamber onto which there is deposited, preferably from the vapor phase, an overlayer of like semiconductor material together with a very closely controlled, predetermined amount of conductivity determining impurities to achieve a desired impurity content proportional to a selected resistivity range. in a subsequent deposition of pure semiconductor material which is effected from the vapor phase the first deposited and precisely doped layer is overgrown and the impurity concentration of the final rod of relatively large diamete is arrived at by dilution in a predetermined manner. Thus no doping impurity is lost and the diluting step becomes one of high precision.

The subsequent deposition of pure semiconductor material can be effected either in the same reactor by simply interrupting the addition of doping impurities or the substrate core may be transferred from the doping reactor to a growth reactor after 2. doped inner layer and potective outer layer has been deposited on the substrate in the doping reactor. The latter practice is always desirable where one wishes to produc semeiconductor material of extremely high purity in one area or zone and has to confine all doping operations to another Zone which is well separated from the first one. Thereafter these rods are float zone refined for a selected number of zone passes at a constant travel rate whereby a certain fraction of the impurities is removed thereby providing a plurality of rods of different resistivity.

Hereinafter the process of the present invention will be illustrated by reference to the formation of single crystals of silicon, although other semiconductor material, such as germanium, and compounds of the group Ill-V elements, may be used as well.

Accordingly, in one embodiment of the present invention, N-conductivity type silicon single crystals having predetermined resistivities in the range 0.1- ohm-cm. are produced by the steps of codepositing polycrystalline silicon from a thermally decomposable source of the material, such as silicochl roform, together within. a predetermined amount of phosphorus, from a vapor source of phosphorus trichloride, in the presence of hydrogen onto thin silicon substrate cores, then diluting the core by further deposition of relatively pure semiconductor material from the vapor phase and subsequently zone refining the doped cores in a controlled manner.

Considering the process of the invention in more detail with reference to the flow sheet of FIGURE 1 and the apparatus of FIGURE 2, there is first provided a pair of elongated silicon cores having a diameter of about 5 mm. as a substrate. Onto this core is codeposite-d a conductivity type determining impurity, such as phosphorus, for N-type, or boron, for P-type, together with a heavy overlayer of silicon. In FIGURE 2 there is shown in highly schematic form a suitable apparatus for carrying out the codeposition process from the vapor phase. The apparatus includes a reaction chamber 2 in which is contained the silicon cores 1. The core is resistively heated through a current source connected to the core through base 3. A source of volatile semiconductor material, such as trichlorosilane, is then introduced into the reaction chamber through inlet 4 by passing hydrogen over liquid trichlorosilane 5. A liquid phosphorus trichloride doping agent dissolved in trichlorosilane is contained in vessel 6. A carefully measured amount of the liquid is then intro duced into the hydrogen-trichlorosilane gas stream. Nozzle 7 serves to keep the gaseous reactants in a turbulent condition while in the reactor.

Once in the reaction vessel, the sil icochloroform and the quantity of phosphorus trichloride are decomposed at a rapid rate onto the heated silicon cores thereby forming an overlayer of polycrystalline silicon uniformly doped with a carefully controlled concentration of phosphorus impurities. in a typical run, for example, 75x10 ml. of phosphorus trichloride dissolved in 12.5 ml. trichlorosilane (5.2 10 atoms P) are added dropwise to a gaseous mixture of one part silicochloroform to nine parts hydrogen at flow rates of 500 g. SiHCl and 12 liters H respectively, in the presence of polycrystalline silicon cores each having a length of about 12 inches and a diameter of 5 mm. The cores are heated at 1150 C. during about 15 hours. During the initial deposition all the phosphorus is deposited. Thereafter the diameter of the core is increased by a predetermined amount controlled by the overall time of deposition at given flow rates in accordance with the resistivity level desired for the material. For example, approximately 400 g. of silicon are deposited. The silicon contains a uniform distribution along the length of the rod of 3 1tl atoms of phosphorus per cc. which is equivalent to a resistivity of 0.2 ohm-cm. This adjustment in impurity content quite nearly determines the resistivity level of the resultant single crystal semiconductor material produced during subsequent steps in the process.

To convert polycrystalline material to single crystal material and to provide a final adjustment in the resistivity level of the material, the doped rods are float zone refined in any conventional vacuum zone refining apparatus well known in the art and described in detail, for example, in U.S. Patent 2,901,325.

In general, float zone refining involves mounting the semiconductor rod in a vacuum with a heating coil around it, and movin" the coil at a prescribed travel rate along the length of the rod. In the present process a standard travel rate of 3 mm. per minute is used. Thereby a decrease in resistivity level of the doped rods is merely a function of the number of zone passes of the coil, namely the number of complete traversals of the length of the crystal by the moving coil.

To illustrate typical results of zone refining, after one pass the resistivity of the resultant single crystal rod was 0.24 ohm-cm; after two passes, 0.36; after three passes, 0.5; after four passes, 0.89; and after seven passes, 4.2.

By adjusting the initial concentration of doped master rods together with a selected number of zone passes single crystal silicon rods having a predetermined resis- :tivity in the range of 0.01-100 ohm-cm. are produced. Below are summarized some data which illustrates the variation of resistivity with the concentration of doping material and the number of zone passes for diameter increases from core to rod of 20.

T able I Measured N Type Resistivity, Ohmem.

No. of Zone Refining Passes While the invention is presently described with particular reference to certain embodiments thereof it will be understood various changes and alterations may be made in accordance with the skill of the art.

I claim:

1. A method of forming a single crystal silicon semiconductor material having uniform predetermined resistivity and N conductivity type within the range 2-66 ohm-cm. which comprises providing a uniform elongated silicon core of given diameter, codepositing silicon and phosphorus onto said core from the vapor phase by thermal decomposition of a vapor formed from 0.2 50 ml. of a solution containing silicochloroform and phosphorus trichloride in a ratio of 6X10 ml. PCl /ml. SiHCl in the presence of hydrogen to produce a predetermined polycrystalline overlayer of silicon together With a predetermined concentration of N conductivity type phosphorus impurities only somewhat above the desired impurity level, and thereafter float zone refining said deposited rod in a vacuum for a selected number of zone passes from 1 to 13 at a constant zone travel rate to reduce the impurity level thereby to convert said polycrystalline deposit into single crystal material and to increase its resistivity to a predetermined level, the number of zone passes determining the degree of increase in the resistivity of said rod.

2. A method of forming a single crystal silicon rod having a resistivity of 0.36 ohm-cm. N which comprises providing a silicon semiconductor rod having a diameter of 5 mm. and a length of 12 inches, heating said rod to 1150 C., introducing a gaseous mixture of one part silicochloroform to 9 parts of hydrogen at a flow rate of 500 g. SiHCl per hour and 12 liters H per minute respectively together with 7.5 10- m1. of phosphorus trichloride, continuing said heating for about 15 hours to produce an overlayer of about 400 g. of silicon together with 3 10 atoms of phosphorus uniformly distributed along the length of the rod, and thereafter float zone refining said rods in a vacuum at a travel rate of 3 mm. per minute during two zone passes to produce thereby a single crystal silicon rod having a resistivity of 0.36 ohm-cm. N.

References flied in the file of this patent UNITED STATES PATENTS 2,739,088 Pfann Mar. 20, 1956 2,910,394 Scott et al. Oct. 27, 1959 2,964,396 Rumrnel et al. Dec. 13, 1960 2,970,111 Hoflman et al. Jan. 31, 1961 3,011,877 Schweickert et al. Dec. 5, 1961 

1. A METHOD OF FORMING A SINGLE CRYSTAL SILICON SEMICONDUCTOR MATERIAL HAVING UNIFORM PREDETERMINED RESISTIVITY AND N CONDUCTIVITY TYPE WITHIN THE RANGE 2-66 OHM-CM. WHICH COMPRISES PROVIDING A UNIFORM ELONGATED SILICON CORE OF GIVEN DIAMETER, CODEPOSITING SILICON AND PHOSPHORUS ONTO SIAD CORE FROM THE VAPOR PHASE BY THERMAL DECOMPOSITION OF A VAPOR FORMED FROM 0.250 ML. OF A SOLUTION CONTAINING SILICOCHLOROFORM AND PHOSPHORUS TRICHLORIDE IN A RATIO OF 6X10**5 ML. PCL3/ML. SIHCL3 IN THE PRESENCE OF HYDROGEN TO PRODUCE A PREDETERMINED POLYCRYSTALLINE OVERLAYER OF SILICON TOGETHER WITH A PREDETERMINED CONCENTRATION OF N CONDUCTIVITY TYPE PHOSPHORUS IMPURITIES ONLY SOMEWHAT ABOVE THE DESIRED IMPURITY LEVEL, AND THEREAFTER FLOAT ZONE REFINING SAID DEPOSITED ROD IN A VACUUM FOR A SELECTED NUMBR OF ZONE PASSES FROM U TO 13 AT A CONSTANT ZONE TRAVEL RATE TO REDUCE THE IMPURITY LEVEL THEREBY TO CONVERT SAID POLYCRYSTALLINE DEPOSIT INTO SINGLE CRYSTAL MATERIAL AND TO INCREASE ITS RESISIVITY TO A PREDETERMINED LEVEL, THE NUMBER OF ZONE PASSES DETERMINING THE DEGREE OF INCREASE IN THE RESISTIVITY OF SAID ROD. 